There are a number of situations in which it is desirable or even necessary to provide a high gain signal repeater. For example, it is now well known that the cell phone signals are weak or nonexistent in a building or other structure such as a large sports stadium which structures are partially or completely enclosed using materials such as steel, aluminum and concrete. In these situations, it is desirable to provide a signal repeater that is designed to receive incoming cell phone signals from a cell phone power, broadcast the signals within the structure and also received outgoing cell phone signals from within the structure to be broadcast back to the cell phone tower.
A problem exists when such a high gain repeater is deployed in a way that coupling can occur between the input and output antennas of such systems. A typical repeater gain may be 85 dB, with antennas separated by a distance of only meters to tens of meters in difficult deployment situations. When two antennas of a bi-directional amplifier (BDA) are so close together, unwanted signal coupling is stronger, and this will cause the amplified signal to be distorted and will potentially cause the BDA to oscillate. The propagation of signals with multipath, usually present, causes a situation where overall gain is equal to or greater than 0 DB, and a phase shift is 0° at a frequency. Under these conditions, an oscillation can occur.
For example, in an in-building deployment, a repeater or antenna may be placed on a roof immediately above a server antenna. If the penetration loss through the roof is not accounted for, a higher than zero DB overall gain is possible and oscillation will likely occur.
Various methods have been used in the past to reduce coupling. These include methods using antenna lobe suppression, shielding material, absorbing foam and cancellation methods. Cancellation methods, especially those using digital techniques, are generally preferred as they involve less in terms of special deployment techniques. One cancellation method that has been used is the Least Mean Square or normalized Least Mean Square method. This technique uses a digital correlator to determine the propagation coupling between the input and output antenna, and an accumulator in a feedback loop to set coefficients in a cancellation filter.
A repeater has unique problems in the presence of strong variable signals, particularly on the uplink. Uplink signals are generated typically by hand sets. When the composite uplink signal is a result of a small number of hand sets, the power can fluctuate a great deal. This is due to the burst nature of most communication protocols, particularly those in TCP/IP based systems. The burst nature of the signals has been known to produce a lack of coverage in LMS and normalized LMS systems, and prevents their use as a good solution to uplink cancellation suppression.
Accordingly, what is needed is an easy to implement and reliable system and method designed to incorporate feedback cancellation in a bidirectional amplifier where there is signal coupling between the input and the output antenna, in order to prevent the bidirectional amplifier from oscillating and to reduce the distortion and modulation errors in the amplified signals caused by regenerative gain unflatness.